Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections. In case of photolithography which can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces of semiconductor wafers on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small. As a means for planarizing the semiconductor wafer, it is polished by a polishing apparatus.
Conventionally, this kind of polishing apparatus has a turntable and a top ring, and the top ring applies a certain pressure to the turntable. The workpiece to be polished is placed between the top ring and the turntable, and while supplying an abrasive liquid, the workpiece is polished to a flat mirror finish.
In the polishing apparatus described above, if the workpiece is not pressed against the polishing cloth under forces which are uniform over the entire surface of the workpiece, then the workpiece tends to be polished insufficiently or excessively in local areas depending on the applied forces. Therefore, in the conventional polishing apparatus, as a means for preventing nonuniform pressing forces, the following arrangements have been proposed.
1 An elastic pad of polyurethane or the like is applied to a wafer holding surface of the top ring for uniformizing a pressing force applied from the top ring to the semiconductor wafer.
2 A workpiece carrier for holding the workpiece, i.e. the top ring, is tiltable with respect to the surface of the polishing cloth.
3 A region of the polishing cloth surrounding the workpiece is pressed independently of the top ring and the workpiece, for thereby eliminating an appreciable step between a region of the polishing cloth pressed by the workpiece and the surrounding region thereof.
FIG. 12 of the accompanying drawings shows a main part of a conventional polishing apparatus. The conventional polishing apparatus comprises a turntable 41 with a polishing cloth 42 attached to an upper surface thereof, a top ring 45 for holding a semiconductor wafer 43 to allow the semiconductor wafer 43 to be rotated and to be pressed, and an abrasive liquid supply nozzle 48 for supplying an abrasive liquid Q to the polishing cloth 42. The top ring 45 is connected to a top ring shaft 49, and is provided with an elastic pad 47 of polyurethane or the like on its lower surface. The semiconductor wafer 43 is held by the top ring 45 in contact with the elastic pad 47. The top ring 45 also has a cylindrical guide ring 46A on an outer circumferential edge thereof for retaining the semiconductor wafer 43 on the lower surface of the top ring 45. Specifically, the guide ring 46A is fixed to the top ring 45, and has a lower end projecting downwardly from the lower surface of the top ring 45 for holding the semiconductor wafer 43 on the elastic pad 47 to prevent removal of the top ring 45 under frictional engagement with the polishing cloth 42 during a polishing process.
In operation, the semiconductor wafer 43 is held against the lower surface of the elastic pad 47 which is attached to the lower surface of the top ring 45. The semiconductor wafer 43 is then pressed against the polishing cloth 42 on the turntable 41 by the top ring 45, and the turntable 41 and the top ring 45 are rotated to move the polishing cloth 42 and the semiconductor wafer 43 relatively to each other, thereby polishing the semiconductor wafer 43. At this time, the abrasive liquid Q is supplied onto the polishing cloth 42 from the abrasive liquid supply nozzle 48. The abrasive liquid Q comprises an alkaline solution containing an abrasive grain of fine particles suspended therein, for example. The semiconductor wafer 43 is polished by a composite action comprising a chemical polishing action of the alkaline solution and a mechanical polishing action of the abrasive grain.
FIG. 13 of the accompanying drawings shows in an enlarged fragmentary cross-section the semiconductor wafer, the polishing cloth and the elastic pad during polishing by the polishing apparatus shown in FIG. 12. As shown in FIG. 13, in case of such structure in which only the workpiece presses the polishing cloth, the workpiece, i.e. the semiconductor wafer 43, has a peripheral portion which is a boundary between contact and noncontact with the polishing cloth 42 and also is a boundary between contact and noncontact with the elastic pad 47. At the peripheral portion of the workpiece, the polishing pressure applied to the workpiece by the polishing cloth and the elastic pad is not uniform, and thus the peripheral portion of the workpiece is liable to be polished to an excessive degree. As a result, the peripheral edge of the workpiece is often polished into a so-called edge-rounding.
In order to prevent the peripheral portion of the semiconductor wafer from being excessively polished, there has been proposed in Japanese patent application No. 9-105252 a polishing apparatus having a structure for pressing an area of the polishing cloth which is located around the peripheral portion of the semiconductor wafer.
FIG. 14 of the accompanying drawings shows the polishing apparatus disclosed in Japanese patent application No. 9-105252. In FIG. 14, the reference numeral 51 represents a top ring which comprises a top ring body 51A and a retainer ring 51B removably fixed to the peripheral portion of the top ring body 51A by bolts 181. A recess 51a for accommodating a semiconductor wafer 54 is formed by a lower surface of the top ring body 51A and the retainer ring 51B. The semiconductor wafer 54 has an upper surface held by the lower surface of the top ring body 51A, and an outer peripheral edge held by the retainer ring 51B. A presser ring 53 is vertically movably disposed around the top ring body 51A and the retainer ring 51B. A leaf spring 67 having a U-shaped cross-section is provided between the presser ring 53 and the top ring 51 to prevent a direct contact of the presser ring 53 and the top ring 51 and to suppress excessive tilting of the top ring 51.
An elastic pad 52 is attached to a lower surface of the top ring 51. A turntable 55 having a polishing cloth 56 attached thereto is disposed below the top ring 51. An attachment flange 182 having a spherical concave surface 182a is fixed to the top ring body 51A. A top ring shaft 58 is disposed above the top ring 51. A drive shaft flange 184 having a spherical concave surface 184a is fixed to the lower end of the top ring shaft 58. A spherical bearing 57 is disposed between the spherical concave surfaces 182a and 184a. A space 183 is formed between the top ring body 51A and the attachment flange 182, and vacuum, pressurized air, and liquid such as water can be supplied to the space 183.
The top ring shaft 58 is coupled to a top ring air cylinder (not shown) fixed to a top ring head 59. The top ring shaft 58 is vertically movable by the top ring air cylinder, and the semiconductor wafer 54 held by the lower end surface of the top ring 51 is pressed against the polishing cloth 56 on the turntable 55. Further, the top ring shaft 58 is coupled to a top ring motor (not shown), and the top ring 51 is rotated by the top ring motor. The presser ring 53 provided around the top ring 51 is coupled at its upper end to presser ring air cylinders 72. The presser ring air cylinders 72 are fixed to the top ring head 59. A plurality of (e.g. three) the presser ring air cylinders 72 are circumferentially spacedly provided. There is no means such as a key between the top ring 51 and the presser ring 53 for transmitting the rotation of the top ring 51 to the presser ring 53. Therefore, the top ring 51 is rotated about an axis of the top ring shaft 58 during polishing, but the presser ring 53 is nonrotatable about its own axis.
The top ring air cylinder and the presser ring air cylinders 72 are connected to a compressed air source (not shown) through respective regulators (not shown). By adjusting the pressing force of the presser ring with respect to the pressing force of the top ring, the distribution of polishing pressures is made continuous and uniform from the center of the semiconductor wafer to its peripheral edge and further to the outer circumferential edge of the presser ring disposed around the semiconductor wafer. Consequently, the peripheral portion of the semiconductor wafer is prevented from being polished excessively or insufficiently.
In the polishing apparatus proposed in Japanese patent application No. 9-105252, the presser ring 53 is supported only by the shafts of the air cylinders 72 fixed to the top ring head 59. Although plural of (e.g. three) the air cylinders 72 are disposed circumferentially at equal intervals, the presser ring 53 is supported by a so-called cantilever structure in which the presser ring 53 relies only on rigidity of the shafts of the air cylinders. Because a large frictional torque is applied to the lower surface of the presser ring 53, the presser ring 53 cannot be highly rigidly, i.e. reliably, supported. Thus, it is difficult to ensure the concentricity of the top ring and the presser ring, and the presser ring is eccentrically positioned with respect to the top ring. Consequently, the full circumferential area of the polishing cloth around the peripheral portion of the semiconductor wafer cannot be uniformly pressed, and polishing effect of the peripheral portion of the semiconductor wafer is adversely affected.
Further, the leaf spring 67 is disposed between the top ring 51 and the presser ring 53 to prevent a direct contact of the top ring 51 and the presser ring 53 and to suppress excessive tilting of the top ring 51. However, as described above, the rigidity of the structure for supporting the presser ring 53 is insufficient, and hence the leaf spring 67 is rapidly worn out and the frequency of replacement of the leaf spring 67 is high.
Furthermore, since the presser ring 53 surrounds the full circumference of the top ring 51 and presses the polishing cloth 56 downwardly during polishing of the semiconductor wafer which is a workpiece, the presser ring 53 prevents the slurry-like abrasive liquid supplied to the surface of the polishing cloth 56 from entering the inside of the presser ring 53. As a result, the abrasive liquid which is located between the polishing cloth and the workpiece and is actually used for polishing is insufficient in quantity.